Naturally occurring gamma radiation for K-U-T (potassium, uranium and thorium) elements yields gamma ray intensity vs. energy spectra in the vicinity of a well borehole observed by highly stable scintillation detectors in the well borehole. The radiation at the scintillation detector and its associated photomultiplier produces a pulse height spectrum proportional to the energy of gamma rays impinging on the scintillation crystal. The spectrum is divided into selected energy ranges or windows. Three windows are centered about selected gamma ray emission peaks for the naturally occurring gamma rays of the K-U-T elements. Gamma ray count rates in each of the three energy ranges are transmitted to the surface and processed by a technique known as spectrum stripping wherein standard calibration spectra, for each of the individual elements (obtained in standard boreholes) are applied to the unprocessed data (count rates) of the selected windows (energy ranges) to detect each of the three elements of interest. The "stripping constants" are derived from measurements of the standard gamma ray energy spectra in standard boreholes containing essentially only one of the three elements to enable the stripping constants to be applied to the measured spectrum in an unknown earth formation surrounding a borehole. The concentrations of the three elements of interest are determined after application of the stripping constants. After carrying out specified procedures, elemental concentrations of the K-U-T elements are obtained. A fourth window is used to compensate the K-U-T concentrations for borehole effects, as described in U.S. patent application filed May 21, 1981, Ser. No. 265,736 now U.S. Pat. No. 4,436,996. Fifth and sixth windows in the observed spectrum are processed to isolate a factor indicative of formation matrix type.
The actual gamma ray count rate achieved at a scintillation detector in a well borehole is dependent on the Compton attenuation coefficient .eta.. Each photon has a point of origination somewhere in the adjacent earth formations in traveling toward the scintillation detector. The attenuation of the gamma ray photon flux along the path of travel is dependent on the thickness of the material, the density of the material and the formation matrix type of the material. The gamma ray photons travel along a path having a length which is statistically determined from the distributed emission sources, namely the K-U-T elements. The present invention provides a measurement of formation matrix type of an adjacent formation by utilization of the measured natural gamma ray spectrum observed at a scintillation crystal coupled with signal processing procedures as described below.
Major attenuation factors of the gamma ray flux include pair production, Compton scattering, and photoelectric absorption. Below certain energy levels, pair production is negligible and, therefore, not significantly involved in the method described herein.
The observed or measured gamma ray energy spectra are thus separated into six energy level windows. The location of the six energy windows in the observed gamma ray spectrum is important. There are three predominant energy peaks for the K-U-T elements, and windows are normally defined to observe the peaks. THe K-U-T peaks are 1.46, 1.76 and 2.61 MeV gamma radiation peaks for potassium (K.sup.40), uranium (Bi.sup.214) and thorium (Tl.sup.208), respectively. A fourth energy window is defined in the Compton dominated spectrum to compensate for borehole and formation density induced changes in the calculated K-U-T concentrations. The fifth window (which may or may not overlap the fourth window) is sufficiently high in energy range to be above the effects of photoelectric absorption so that the primary mode of photon attenuation is Compton scattering. The sixth window is defined at very low gamma ray energy levels where photoelectric absorption is of major importance and relative attenuation due to pair production or the Compton effect is minimized.
The measured count rates in the fifth and sixth windows can be used to define a ratio which, after isolation of K-U-T elemental concentration effects, is primarily a function of the formation photoelectric absorption cross-section U, and the borehole parameters adjacent to the tool.